The present invention relates in general to humidity sensors of the type having a probe that is inserted into the soil and is concerned more particularly with an improved electrical circuit for indicating the moisture or humidity condition of the soil. The invention has its preferred application in monitoring soil for plants to enable the humidity of the soil to be kept at a level that is proper for the plants.
The proper watering of house plants is not easily accomplished and many times the plants are over-watered or under-watered. The difficulty is due to the many different variables that affect the moisture content of the soil in which the plant is disposed. For example, in the case of potted plants in a house, soil moisture can be affected by the size of the pot, the relative humidity of the air in the room, the type of soil used, and the amount of light the plant receives. "Prior art" soil moisture indicators such as the one shown in U.S. Pat. No. 3,979,667 employ a probe having two terminals for sensing the resistance of the soil to current flow between the terminals. Although such devices function adequately as indicators, their manufacturing cost tends to be somewhat high because they require at least two separate transistors and associated biasing resistors.
Accordingly, the principal object of the present invention is to provide a low cost soil moisture indicator that uses fewer components than "prior art" indicator devices and whose performance is as good as or better than the performance of the "prior art" devices.